An ill-defined term that means many things to many people.
Complex things are neither random nor regular, but hover somewhere in between. Intuitively, complexity is a measure of how
interesting something is. Other types of complexity may be well defined; see the index for other references.mitpress.mit.edu/books/FLAOH/cbnhtml/glossary-C.html

Complexity tends to be used to characterize something with many parts in intricate
arrangement. The study of these complex linkages is the main goal of network theory and network science.

..There are two kinds of complexity. Detail Complexity is
when there are many variables. Dynamic Complexity is situations where cause and effect are subtle, and where
the effects over time of interventions are not obvious.